1. Field of the Invention
The present invention relates to manufacturing processes that require delivery of highly accurate amounts of gas to a processing chamber. More particularly, the invention concerns an improved gas flow system that accurately measures gas flow during the delivery of gas to a processing chamber. Added operations may be performed to regulate gas flow in accordance with these measurements.
2. Description of the Related Art
Many industrial processes, such as semiconductor manufacturing, rely on the accurate delivery of gasses to processing chambers, which are also called "reaction vessels." These chambers operate at various pressures, ranging from very high pressures in some cases to very low pressure in others. The accuracy and stability of the gas delivery system is critical to the overall manufacturing process. The chief goal of these systems is to accurately deliver a prescribed mass of gas. Since the relationship between mass and volume is not constant, and depends on other factors, purely volumetric flow control devices are not particularly useful.
Historically, engineers have used thermal mass flow controllers to control the flow of process gasses. In a complete gas delivery system, these thermal mass flow controllers are present in conjunction with various filters, pressure transducers, and control valves. These components are typically connected with steel tubing and various mechanical couplings. Typical connection schemes include welding, brazing, and various reusable fittings. Such fittings employ elastomeric or metal seals held in compression to form a vacuum-tight mechanical seal.
FIG. 1 shows an exemplary thermal mass flow controller 100. Gas first enters a gas inlet 102, and thereafter takes a flow path 103. After the inlet 102, gas flows around a bypass restrictor 104. Due to the pressure drop developed across the bypass restrictor 104, a fixed percentage of gas is diverted through a capillary tube 106, in a flow path 107. A multi-stage heater winding 105 is wrapped around the capillary tube 106. The winding 105 includes multiple terminals 105a-105c, which number three in this example. As the gas exits the capillary tube 106, it rejoins the main gas stream 108 to form a combined flow 111 that continues to a control valve 112. The control valve 112 includes valve windings and magnetics 114 and a plunger 116. The position of the plunger 116 regulates the amount of gas flow through the mass flow controller. Wider plunger settings permit more gas flow, whereas more constricted plunger settings permit less gas flow. Control electronics 122 regulate plunger position to achieve a desired gas flow, as described below. After the control valve 112, gas flows in a path 118 that finally exits the mass flow controller 100 at a gas outlet 120. The gas outlet 120 may lead to a processing chamber via further "downstream" plumbing (not shown).
The mass flow controller 100 works on the following principle. The mass of fluid flowing through the capillary tube 106 (flow 107) is directly proportional to the amount of flow around the bypass restrictor 104 (flow 108), and therefore provides a representative measure of the total flow through the device. Thus, the mass of gas in the flow 107 multiplied by a fixed number equals the mass of the gas in the flow 108. The sum of gas flows 107 and 108 equals the gas flow 103. The mass flow controller 100 may be manufactured for a specific flow range, taking into account relative size and configurations of the capillary tube, bypass flow path and control valve.
According to one method of mass flow measurement, electrical current is passed through the heater windings 105 from terminal 105a to terminal 105c. The resistance of the heater windings 105 changes in a known way with temperature. Thus, when the winding 105 is operated in a Wheatstone bridge, the terminal 105b can be used as a measurement point. As gas flows through the capillary tube 106, the thermal mass of the gas transfers heat from the first part of the winding (between terminals 105a-105b) to the second part of the winding (between terminals 105b-105c). The amount of mass flow determines the amount of thermal transfer, which results in a directly proportional voltage imbalance between the winding 105a-105b and the winding 105b-105c. This voltage imbalance represents the amount of mass flowing in the tube 106. With knowledge of the amount of mass passing through the capillary tube 106, the total amount of mass in the flow 103 is easily calculated as discussed above.
Different variations of this measurement principle have also been used. For instance, a single heater winding and two temperature measurement devices may be used to measure the thermal transfer due to flow. As another alternative, a variable amount of current may be directed through one or both of the heater windings in order to maintain a fixed temperature drop along the capillary tube due to flow.
In operation, control electronics 122 regulate plunger positioning under a closed loop feedback system. Namely, the electronics 122 compare detected mass flow (measured by the capillary tube 106) to desired mass flow (provided as input). Then, based on this comparison, the electronics 122 responsively narrows or opens the plunger 116 position.
Mass flow controllers are one of the most important parts of gas delivery systems. Unfortunately, known mass flow controllers can also one of the least reliable parts of such a system. Mass flow controllers have been manufactured with many different configurations of capillary tubes, windings, bypass restrictors, and control valves. Nonetheless, several different factors cause undesirable variations in mass flow calibration and performance. If any liquid or other contamination forms in the area around the bypass restrictor, the relationship between the flow 107 and the flow 103 varies, and the overall calibration of the device changes. Condensation forming in the bypass flow path or elsewhere in the flow path is another source of calibration error. Aging of the windings and the nature of the thermal contact between the windings and the outside of the tube cause long term calibration drift. Changes in chemical composition of the process gas as it is subjected to the winding heat can also affect the integrity of the process.
Another flow rate regulation system appears in U.S. Pat. No. 4,285,245 to Kennedy. Kennedy measures the pressure decrease in a measurement chamber of fixed volume, and calculates the rate of pressure decrease by dividing the measured pressure drop by time of drop. This calculated rate of fall is directly related to the volumetric flow rate. Although the Kennedy system may be useful for its intended purpose, it may prove inadequate for applications seeking to precisely control the mass flow rate. In particular, the mass of a gas is not always proportional to its volume, since this relationship can change under the influence of factors such as absolute pressure and temperature. Also, small incremental variations in mass flow rate can occur undetected in the Kennedy system because, as recognized by the present inventors, Kennedy lacks any continuous or real-time measurement and flow control means. Thus, the Kennedy approach may not be satisfactory for applications that seek to precisely control mass flow.
In the semiconductor manufacturing line, misdelivery of process gasses can be extremely costly. In some cases, if the process gas is incorrectly delivered to a silicon wafer in the process chamber, the wafer may be ruined. And, since economy warrants growing larger and larger silicon ingots, these large silicon wafers are more costly to scrap if damaged. Furthermore, in the event of such an error, its is expensive to repair or replace the mass flow controller and repeat the manufacturing run. In many cases, manufacturing downtime can result in lost revenues exceeding $125,000 per hour.
In view of these limitations, known mass flow controllers are not completely adequate for some applications due to certain unsolved problems.